help with -y passwdfile argument

hi,
i am using openldap-2.2.23 and i try to write a script using the command
line argument/option for ldapsearch -y passwdfile.
but when i try the option -y passwdfile i get always an error:
ldap_bind: Invalid credentials (49)
i already tried different content for the passwdfile, like the plaintext
password alone with and without new line and tested the whole with
really simple passwords. the passwdfile is readable.
if i test the same command with the options -W or -w password, then
there is no credentials error and i get the expected result.
examples of the commands used:
ldapsearch -x -D 'cn=Manager,dc=example,dc=com' -y ./passwdfile -b
'dc=example,dc=com'
ldapsearch -x -D 'cn=Manager,dc=example,dc=com' -W -b
'dc=example,dc=com'
ldapsearch -x -D 'cn=Manager,dc=example,dc=com' -w secret -b
'dc=example,dc=com'
so, my question is: what is the format of the entry in the passwdfile
for this to work?
any hints?
TIA, manu.